A wireless communication device such as a mobile phone terminal comprises a radio frequency power amplifier for amplifying a modulated signal in its transmission circuit. When the output load of the radio frequency power amplifier fluctuates for some reason and deviates from 50 ohms, its output power increases. In other words, as is well known, the load power is at its maximum when the output impedance of the signal source and the input impedance of the load are matched at 50 ohms. When the input impedance of the load deviates from 50 ohms, the load power decreases and the power consumed by the signal source increases. The increase in output power due to the fluctuation of the output load causes an increase in the collector current and voltage of an amplifying transistor in the output stage of the radio frequency power amplifier, and may result in the destruction of the amplifying transistor in the output stage. As an example of a radio frequency power amplifier having the function of protecting the amplifying transistor from overcurrent and overvoltage in such cases, a structure shown in FIG. 5 is known (refer to FIG. 1 of Patent Document 1).
In FIG. 5, reference symbol 4 indicates the structure of a semiconductor device (IC chip) as a radio frequency power amplifier transistor chip. This IC chip 4 comprises a radio frequency power amplifier transistor 3 (also known as “amplifying transistor”) and a protection circuit, and the protection circuit is composed of a voltage detection circuit 1 and a shunt circuit 2. A radio frequency matching circuit 7 for matching input impedance and a radio frequency matching circuit 8 for matching output impedance are provided at an input side terminal 5 and an output side terminal 6 of the IC chip 4, respectively. These are disposed on the same substrate and constitute a radio frequency power amplifier module 9. The emitter of the amplifying transistor 3 is connected to a ground terminal 11 of the module 9 via a ground terminal 10 of the IC chip 4. The voltage detection circuit 1 is constituted by a plurality of vertically (in-series) connected diodes 12 composed of NPN transistors whose bases and collectors are short-circuited, the collector of the uppermost diode 12 is connected to a collector voltage terminal 13 of the amplifying transistor 3, the emitter side of the lowermost diode 12 is grounded via the ground terminal 10 of the IC chip 4, and the short-circuited base and collector of the lowermost diode 12 form an output terminal of the voltage detection circuit 1. The shunt circuit 2 is composed of an NPN transistor (switching transistor) 14 whose collector is connected to a base voltage terminal 15 of the amplifying transistor 3, whose base is connected to the output terminal of the voltage detection circuit 1, and whose emitter is grounded via the ground terminal 10. A base bias terminal 16 and a collector voltage terminal 17 of the radio frequency power amplifier module 9 are connected to the base voltage terminal 15 of the transistor 3 and the collector voltage terminal 13 of the transistor 3, respectively. An input terminal 18 connected to the radio frequency matching circuit 7 for matching input impedance of the radio frequency power amplifier module 9 is provided on the input side, and an output terminal 19 connected to the radio frequency matching circuit 8 for matching output impedance is provided on the output side. An external load 20 of 50 ohms is connected to the output terminal 19.
FIG. 6 shows the voltage-current characteristics (characteristics when the base current is varied in step) of the amplifying transistor 3 in FIG. 5 (refer to FIG. 5 of Patent Document 1). The abscissa indicates the collector voltage (the collector-emitter voltage) of the amplifying transistor 3 and the ordinate indicates the collector current. In FIG. 6, the area of safe operation (ASO) of the transistor is indicated by a bold line; the collector voltage and collector current should not exceed the area indicated by the bold line. It is desired that a protection function is provided so as to decrease the collector current for protecting the transistor when an overvoltage causing destruction is applied to the collector of the transistor.
In the conventional radio frequency power amplifier shown in FIG. 5, the in-series-connected diodes 12 turn on when an overvoltage is applied to the collector voltage terminal 13 of the amplifying transistor 3, and the NPN transistor 14 turns on. Then the current flowing through the base of the amplifying transistor 3 is shunted to the GND side and the base current of the amplifying transistor 3 decreases, reducing the collector current and protecting the amplifying transistor 3 from being destructed.
[Patent Document 1]
Japanese Patent Kokai Publication No. JP-P2003-347857A
The entire disclosure of the foregoing patent document is incorporated herein by reference thereto.